Protective circuit for loudspeaker



United States Patent Inventor Sidney A. Corderman Binghamton, New York 810,706

March 26, 1969 Dec. 1, 1970 McIntosh Laboratory, Inc. Binghamton, New York a corporation of Delaware Appl. No. Filed Patented Assignee PROTECTIVE CIRCUIT FOR LOUDSPEAKER 7 Claims, 2 Drawing Figs.

US. Cl. 179/1; 307/93, 307/914, 307/248; 323/22 Int. Cl. H04r 3/00; H02h 3/20; GOSf 1/60 Field of Search 179/1 (A), 1, 1(SW); 307/248, 299, 93, 9 4, 100; 330/144; 323/22, 66.9

AMP

[56] References Cited UNITED STATES PATENTS 3,058,010 10/1962 S. C. Rockafellow 307/248 3,206,695 9/1965 E. Y. Bennett, Jr. 307/93X 3,218,542 11/1965 V. L. Taylor 323/22 Primary Examiner- Kathleen I-l. Claffy Assistant Examiner-Charles W. Jirauch Attorney-Hurvitz, Rose & Greene ABSTRACT: A protective circuit for a transistor power amplitier which connects both positive and negative voltages directly to a loudspeaker, in which a voltage sensitive bilateral switch is connected to the speaker terminal in series with an ac. filter, to act as a switch, sensitive to voltage of either polarity above a predetermined value, and which when fired fires a triac connected directly across the speaker.

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- lll SIDNEY HCORDERNHN M 6a 0 ATTORNEY PROTECTIVE CIRCUIT FOR LOUDSPEAKER BACKGROUND OF THE INVENTION Transistor power amplifiers commonly are connected directly to a loudspeaker. They are operated push-pull, one transistor being connected to a positive supply and the other to an equal negative supply. If a transistor shorts, on either side, one side of the power supply or the other is connected directly to the loudspeaker, through the shorted transistor. Under these conditions sufficient current flows to the loudspeaker to destroy or seriously damage the loudspeaker and may cause the speaker cone to catch fire. A simple fuse does not provide adequate protection because it acts too slowly, and a supplementary rapidly acting device is therefore needed. According to the present invention, solid state devices are utilized for the purpose, which are connected directly to the loudspeaker system and which are capable in response to control voltage of either polarity of firing and conducting full load current around the loudspeaker to ground.

SUMMARY OF THE INVENTION BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a schematic circuit diagram of a system according to the invention; and t FIG. 2 is a schematic circuit diagram of a modification of the system of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Tenis an amplifier circuit antecedent to an output stage consisting essentially of two series connected NPN transistors T, and T The collector of T, is connected via a fuse F, to a positive voltage supply terminal 11. The emitterof T is connected via a fuse F to a negative voltage supply terminal 12. The emitter of T, is connected via a very small resistance 13 to output terminal 14. The collector ofT, is similarly connected. The amplifier configuration is wholly conventional. A loudspeaker L is connected between terminal 14 and ground.

The output terminal 14 is connected directly to an anode I of triac TR and via a 1K resistance 16 to the gate electrode of triac TR, anode 2 of which is grounded. C constitutes a low pass filter so that the audio voltage across the $88 or bilateral switch S is always zero, absent d.c. voltage, since one side of S is connected to the junction of C and R and the other side to the terminal 14 via small resistance (1K) 16. However, if either T, and T shorts, high d.c. voltage from either terminal 11 or 12 will be applied to one side of S and the other side will be at ground voltage via resistance R, and the SBS will fire.

The $38 is a silicon planar monolithic integrated circuit having the electrical characteristics of a bilateral thyristor, but which switches rapidly at :8.v. It is normally supplied with a gate lead, not used in the present application.

A triac TR is connected between terminal 14 and ground, and has a gate electrode connected directly to the ungrounded side of S. So long as S is unfired, there is no net gating voltage. When S fires, one side of TR is at supply voltage and the gate electrode, which is on that side, is brought to ground via S and R. TR then fires and having fired remains fired, shorting the loudspeaker L to ground.

In FIG. 2, the terminal 14 is connected to one side of TR, the other side of which is grounded. Resistance R is connected to one side of C, the other side of which is grounded, and S is connected between the junction of C and R and the gate electrode of TR. Resistance 16 is connected in shunt to TR.

In operation, TR is normally unfired, and CR provides a low pass filter which blocks audio frequencies, so that both sides of S are normally at ground potential. If a transistor shorts out, high d.c. voltage is applied to one side of S, which fires, causing d.c. current flow in resistance 16. The resulting d.c. voltage across resistance 16 is applied to the gate electrode of TR, causing it to fire. The resistance 16 also prevents triac turn on due to rate-effect. R is selected to allow adequate triac gate current to assure turn on, and C is then designed in view of the value of R to provide the desired filter characteristics.

The triac is an ac. switch, i.e., a bidirectional triode thyristor which may be gate triggered from a blocking to a conducting state for either polarity of applied voltage, and is roughly equivalent to two SCRs connected in inverse parallel. Once fired, it remains fired until its primary circuit is opened or primary current reduced to zero, i.e., the gate electrode controls firing but not defiring.

Fuses F, and F will blow after the triac has fired, because fuses are slowly acting devices. The system thus provides rapidly acting protection, operation pending blowing of fuses, which are slow acting.

While the is system is applied to protection of a loudspeaker, it is applicable to any type of load.

lclaim:

1. A solid state protective circuit for an audio speaker connected between a terminal and ground, comprising:

a bilateral solid state switch;

a triac connected between said terminal and ground;

said triac having a gate electrode means connecting said bi lateral solid state switch in series with said gate electrode; and

an audio filter connected, arranged, and adapted to supply only d.c. voltage across said bilateral solid state switch and to prevent access to said bilateral solid state switch of ac. voltage in the audio frequency range.

2. A protective circuit for a transistor audio power amplifier having one transistor connected to a positive voltage terminal and another transistor connected to a negative voltage terminal, and a speaker connected to a terminal at the junction of said transistors, comprising:

a bilateral gate electrode controlled solid state switch connected across said speaker;

a bilateral voltage sensitive switch connected between said terminal and said gate electrode; and

an audio filter having a cutoff at the lowest audio frequency of the audio frequency band passed by said audio power amplifier and connected, arranged, and adapted to maintain substantially zero voltage across bilateral voltage sensitive switch in response to said audio frequency band at said terminal but full d.c. voltage in response to d.c. voltage at said terminal.

3. The combination according to claim 2, wherein said filter is a low pass filter having a series resistance and a shunt capacitor connected between said terminal and one side of said bilateral voltage sensitive switch, the other side of said bilateral voltage sensitive switch being connected to said gate electrode.

4. The combination according to claim 2, wherein said filter includes a capacitor connected between said terminal and one side of said bilateral voltage sensitive switch, a resistance connected between said terminal and the other side of said bilateral voltage sensitive switch, said gate electrode being connected directly to said other side of said bilateral voltage sensitive switch, a resistance connecting said other side to ground, and a direct connection to ground from said terminal via the anodes of said solid state switch.

5. A solid state protective circuit for a transistorized audio power amplifier having a speaker connected directly to a power output transistor, of said power amplifier, said transistor normally providing only audio frequency signal to said speaker and providing only high d.c. voltage to said speaker on short circuiting of said transistor, comprising:

a voltage sensitive normally open solid state switch;

6. The combination according to claim 5, wherein said filter means is a low pass audio filter connected in series with said switch.

7. The combination according to claim 5, wherein said switch is an ac. switch and said gate controlled switching device is a triac, and said dc. voltage can be of either polarity. 

